What's a word for lack of confidence?

According to OED:

diffident: modest or shy because of a lack of self-confidence: a diffident youth.

Another word as @Jim suggested would be timid.

timid: showing a lack of courage or confidence; easily frightened: I was too timid to ask for what I wanted.

"Insecurity" would be a good one. To have insecurity is to have lack of self-confidence, lack of self-worth. Insecure.

You can use:

1. Timid: Showing fear and lack of confidence
2. Diffident: Lacking self-confidence. E.g: stood in the doorway diffident and abashed
3. Fainthearted: Lacking conviction or boldness or courage.

I think wavering (“Fluctuating; being in doubt; undetermined; indecisive; uncertain; unsteady”), faltering (To “be unsure or weak” or “Move hesitatingly, as if about to give way”, etc.), and irresolute (“Undecided or unsure how to act” or “Indecisive or lacking in resolution”) might serve.

Also consider unsure, doubting or doubtful, iffy, chancy, and dicey (“Of uncertain, risky outcome” or “Of doubtful or uncertain efficacy, provenance, etc.; dodgy”).